Thesis Defense: Tian Dong, Ph.D. Candidate
Defense Date: Wednesday, April 1st, 2020
Time: 2:00 p.m.
Zoom Meeting Link: https://riceuniversity.zoom.us/j/4708228396
Dynamics of delta building across different scales as informed by the Selenga River Delta, Russia
This dissertation examines dynamics of delta building across different spatial and temporal scales using field measurements and remotely-sensed data collected from the Selenga River delta, Russia. The research topics include evaluating the controls on hydraulic channel geometry for sand-bed rivers, developing a framework to better predict flow partitioning in delta channel networks, as well as deciphering the impacts of tectonic subsidence and variable basin depth on delta lobe building. Field measurements include detailed hydrological and sedimentological data collected within the distributary network of the Selenga Delta over four field campaigns, conducted 2013−2018. Remotely-sensed data include measurements of shoreline position, topset elevation, and basin depth since 1862. This research shows that, for the smallest of channel scales, bankfull width is directly, and depth is inversely, related to the characteristic median bank sediment size. Furthermore, new predicative relations of hydraulic channel geometry for sand-bed rivers that consider bank sediment size are derived, which improve upon previous work. On the channel network scale, channel geometry and planform variables, including cross-sectional area, depth, width, length, and sinuosity are determined to be the best parameters for predicting water and sediment partitioning in a delta system. The best predictive relations for water and sediment partitioning (channel width, length, and sinuosity) are incorporated into an exiting graph theory model, to yield a framework that accurately predicts flux distribution in delta networks. This method is tested by predicting flow partitioning in the Wax Lake delta, the Selenga River delta, and the Lena River delta. Predictions are in good agreement with field measurements. At the delta lobe scale, discrete tectonic subsidence events caused by the Baikal Rift are found to modify avulsion dynamics by varying receiving basin depth. These subsidence events create shallow embayments around the delta, by downdropping a portion of the topset below the average channel depth. As a result, flow is attracted to the partially subsided lobe, as the regional gradient is increased, due to a reduction in delta topset radius. Incisional terraces form as channels re-equalibrate to this new topset gradient. Theory predicts that these terraces reduce the frequency of lobe-scale avulsions by increasing the avulsion-setup threshold. Downstream of the terraces, smaller channel-scale avulsions still occur, and distribute water and sediment to topographic lows between the active channels. Predictive relations and models developed from the studies of this dissertation can be used standalone or incorporated into numerical models to better forecast the response of deltas to perturbations, such as the ongoing climate.